Portable scales for use in measuring large loads such as heavy vehicles require that they be relatively light weight with a small size and a low profile. These type of scales are especially useful by state agencies "in the field" to determine if commercial vehicles are operating within allowable load limits. To accomplish the weight measurement, a number of scales are normally placed on the roadway or the shoulder thereof and the vehicle driven thereon such that all of the vehicle's wheels are supported. Under normal circumstances, the scales may be moved to many different locations during the course of a working day and a large number of different vehicles measured.
The environment in which this weight measurement is performed requires reasonably stable temperature characteristics for the scales in order to obtain an accurate measurement. Normally, prior art scales have provided some type of zeroing mechanism to compensate for temperature variations after the scale has stabilized at an ambient temperature. However, this mechanism requires that the scale be allowed to stabilize for some duration of time prior to performing the weight measurement. These types of scales are impractical in a normal working environment since the scale may be removed from one temperature medium and suddenly placed in another temperature medium, such as from the trunk of a car to a concrete road, and be allowed to stabilize for a very short time before making the weight measurement, resulting in substantial inaccuracies. PG,4
Previously developed portable scales have utilized some arrangement of springs or hydraulics to measure vehicle weight and still retain some level of portability. In the past, such apparatuses have often been bulky in order to achieve the required degree of accuracy. One device that has provided a relatively small size and low profile is disclosed in U.S. Pat. No. 4,007,800 issued to W. Janach, et al. This scale utilizes a plurality of parallel elliptically shaped hollow spring elements. The spring elements all communicate with a gauge and are filled with a fluid. A plate disposed on top of the spring elements compresses the spring elements when subjected to an external weight. The compression of these spring elements results in a volumetric displacement thereby forcing the fluid therein to be displaced into an external reservoir, which is comprised of an expanding bellows type gauge. The deflection of the bellows is proportional to the deflection of a needle on the gauge. By measuring the volumetric change, the elastic deformation of the spring elements can be converted to a weight measurement.
Although the Janach patent discloses a low profile scale, the use of the elliptical spring elements presents manufacturing and cost problems. This is due to the fact that the elliptical spring elements are essentially tubular members that must be carefully dimensioned and attached to a base on the exterior thereof and then the interiors thereof must be attached to a common channel connecting them to the gauge. In addition, the temperature stability of such previously developed scales has not been completely satisfactory. There thus exists a need for a light weight portable scale with improved accuracy and temperature compensation and that is relatively simple to manufacture.